Secondary batteries are essential parts of moving motors such as portable electron equipments and electric motor vehicles as power sources thereof. These secondary batteries generate electrochemical reactions to obtain an electric energy. So, the properties of facilitating the generation of electrochemical reaction, that is activity, greatly affects various battery performance such as the discharge capacity, output characteristic, cycle charging and discharging characteristic, and safety. Accordingly, the activity of the battery can be used as the index of various battery performance. If the initial activity, for example, of the secondary battery can be detected, it can be known whether the secondary battery exhibits desired battery performance or not, and it can be judged whether the battery is inferior or not.
The secondary batteries, such as nickel-hydrogen batteries, may not exhibit a high initial activity, because electrodes thereof do not sufficiently react with electrolytes just after production thereof, so that the potential battery performance thereof may not be obtained. To overcome this problem, these secondary batteries have been charged or discharged prior to using thereof, and consequently the activity thereof has been increased until a required battery performance can be effected.
After the secondary batteries, such as nickel-hydrogen batteries, were produced, they have been initially subjected to a predetermined number of charging and discharging cycles from a fully charged condition to the discharged condition with a predetermined final discharge voltage until the initial capacity activity (potential discharge capacity/theoretical discharge capacity) increases to a predetermined standard or more, before shipping or practically using thereof.
However, when the predetermined number of charging and discharging cycles are performed, as described above, the initial capacity activity of almost all secondary batteries reached a satisfactory level, but the maximum power density (W/kg) which can be discharged by the secondary batteries did not partly reach a required level.
With respect to the secondary batteries of which the initial activity after production is low, it cannot be confirmed before performing the charging and discharging operation whether a required battery performance is effected or not. Accordingly, conventionally, inferior secondary batteries have been also required to be charged and discharged.
If the initial activity can be known, it can be judged before using whether the secondary battery is inferior or not, and consequently it becomes unnecessary to charge and discharge the inferior secondary batteries. This results in the overall production costs of the secondary batteries being decreased by the production costs which have been conventionally needed to charge and discharge the inferior secondary batteries.
On the other hand, when the power of the secondary battery decreases in the course of the motor device being driven by the secondary battery, there occurs the problem that the motor device cannot be driven with high performance. In particular, in the above-described moving motor, it is difficult to supplement power thereto by another power source during driving thereof. So, a secondary battery capable of constantly supplying necessary power has been required.
The secondary battery, however, cannot supply identical power constantly. The supplying power varies with the number of using times. More specifically, as the charging and discharging cycle of the secondary battery is repeated, the electrodes, electrolyte or the like are degraded to gradually decrease the discharging capacity of the battery. Thus, battery degradation occurs and the battery performance decreases. As described above, the battery performance of the secondary battery is lowered due to a large number of using thereof. Consequently, it has become impossible to generate necessary charging and discharging even by applying prescribed charging operation to the secondary battery. Thus, the supplying power decreases. At last, the battery life has ended to require changing thereof.
The secondary battery which has been degraded with the repetition of charging and discharging cycle may be changed to a new one after required battery performance has not been effected thereby, but in accordance with their use like the preceding moving motor, the secondary battery may have to be changed to a new one before required battery performance has not been effected thereby. In this case, the degraded condition of the secondary battery must be determined before the required battery performance is not effected.
In order to supply a sufficient amount of electric energy to a motor device driven by a secondary battery in a proper time period, it has been demanded to provide the method capable of determining the battery condition of the secondary battery as the power source thereof, particularly the activity thereof as required at any time.
With one example of the method for determining the degraded condition of the secondary battery, it can be considered to forecast the degradation time of electrodes, electrolytes or the like before using the secondary battery. However, since the degradation manner of the electrodes and electrolytes or the like, and the process to the degradation thereof depend on the using conditions of the secondary batteries. Accordingly, the degradation time is extremely difficult to forecast beforehand.
To solve this problem, it is possible to calculate the capacity degradation (1-(potential discharge capacity/theoretical discharge capacity)) and determining the degradation condition from the obtained capacity degradation. This method, however, has the defect that degradation of the outputable discharge power, that is output degradation, becomes great even when the capacity degradation is not so great, to prevent the output of a desired power.
Under the above circumstances, conventionally, the internal resistance has been mainly used as the index of the degradation and initial activity of the batteries. For example, as is disclosed in Publication of unexamined patent application No. Hei 7-29614, there has been widely known the method of measuring the current and voltage of the secondary battery (storage battery), obtaining the internal resistance (DC-IR characteristic) from their relation (incline of I-V line), and judging the initial activity and degradation of the secondary battery based on the obtained internal resistance.
However, by merely measuring the internal resistance, the battery condition cannot be sufficiently understood. If a high internal resistance is measured, for example, the reason therefor has been not understood. In addition, in the case the discharge output of the battery being not obtained sufficiently, conventionally, there has been a problem that the reason therefor is not clear, and it is difficult to increase the discharge output. For example, if the battery output is small, because the internal resistance is great though the open voltage of the battery is sufficiently great, it has been difficult to judge whether the increase of the internal resistance results from inferior welding of members such as electrode of the battery, for example, which is impossible to recover, or the initial activity which is able to be overcome by repeated charge and discharge cycles. Furthermore, with this method, the judgement of the degradation of the battery is possible, but the manner of degradation (degradation mode) cannot be judged.
Furthermore, with the method for judging the degradation and initial activity of the secondary battery, which is disclosed in the above-described publication, the variations of both voltage and current must be measured. Consequently, there occurs the problem that the time and cost for measuring the variations of voltage and current are both increased.
In addition, for judging the initial activity and degradation of the battery based on the initial capacity activity and capacity degradation, the battery which has been fully charged under predetermined charging conditions must be discharged completely under predetermined discharging conditions, and then the discharge capacity of the battery must be actually measured. This measurement is troublesome and requires a long period of time. Additionally, there arises another problem that this completely discharging of the battery accelerates the degradation of the battery.
On the other hand, many nickel-hydrogen batteries include a positive electrode which uses nickel oxide or the like as a positive electrode active material, a negative electrode which uses a hydrogen-occluding alloy as a negative electrode active material, and an electrolyte interposed between the positive electrode and negative electrode.
In these nickel-hydrogen batteries, the electrolyte may be dried up during using thereof to deteriorate the battery performance. And as the charging and discharging cycle is repeated may times, the surface of the negative electrode (negative electrode alloy) may be oxidized to degrade the negative electrode, thereby lowering the battery performance.
The manner of degradation (degradation mode) of the secondary battery generally depends on the using conditions thereof. When the nickel-hydrogen battery, for example, is used in electric cars or hybrid cars at about a normal temperature, the negative electrode thereof is gradually oxidized to be degraded. On the other hand, the nickel-hydrogen battery is used in an environment of which the temperature varies greatly to an extremely high temperature, the battery is dried up to be degraded.
Where the battery performance is lowered due to drying of the electrolyte, it can be recovered readily by supplementing the electrolyte. On the other hand, where the battery performance is lowered due to the oxidization of the surface of the negative electrode, it can be recovered by changing the degraded negative electrode for a new one. The hydrogen-occluding alloy for use as the negative electrode active material, however, is relatively expensive, so that changing costs of batteries may increase. Consequently, there has been demanded to provide a method for recovering the battery performance without changing the degraded negative electrode for a new one when the negative electrode is degraded and the battery performance is lowered.
There has been known the method for recovering the battery performance of lead acid batteries by adding a reducing agent to an electrolyte thereof when electrodes thereof are oxidized and the battery performance is lowered (Publication of unexamined patent application No. Sho 53-43842). Similarly, to recover the battery performance of the nickel-hydrogen batteries, which is lowered due to the degradation of the negative electrodes thereof, there can be contemplated the method of adding a reducing agent to electrolyte thereof to reduce the surface of the negative electrodes. This regenerating method, however, causes the reduction of the positive electrodes as well as the negative electrodes, and consequently causes the lowering of Ni valance number so that the battery performance may be lowered.
The present invention has been made in consideration of the above circumstances, and a first object of the present invention is to provide a method and device for judging the condition of secondary batteries, which are capable of judging the same more quickly and in more detail, as compared to the conventional method and device.
A second object of the present invention is to provide a method and device for judging the condition of secondary batteries, which are capable of judging the level of the degradation and initial activity thereof in detail and quickly.
And a third object of the present invention is to provide a method for regenerating secondary batteries such as nickel-hydrogen batteries, which is capable of regenerating degraded secondary batteries properly in accordance with the degraded condition thereof.